Neural network models offer a totally new approach to intelligent information processing that is robust, fault tolerant, and can be extremely fast. These features originate directly from the massive interconnectivity of neurons (the decision-making elements) in the brain and its ability to store information in a distributed manner as a large number of synaptic interconnects of varying strengths. Hardware implementations of neural net concepts, therefore, are attracting considerable attention. Such artificial neural networks are expected, for example, to function as high speed, content addressable, associative memories in large knowledge bases for artificial intelligence applications and robotics or to perform complex computational tasks such as combinatorial optimization for autonomous systems.
Non-volatile, associative electronic memories based upon neural network models, with dense synaptic interconnection arrays in thin-film form, have recently been developed, using hydrogenated amorphous silicon (a-SiH) and manganese dioxide (MnO.sub.2). Irreversible memory switching in hydrogenated amorphous silicon (OFF.fwdarw.ON) and manganese dioxide (ON.fwdarw.OFF) makes them ideally suited for use as programmable, binary, weak synaptic connections in associative programmable read-only-memories (PROMs) based on neural networks models. Although successful, it would be highly desireable to obtain the equivalents of erasable PROMs (EPROMs), such that the switching state can be reversed when desired (ON.rarw..fwdarw.OFF.rarw..fwdarw.ON etc.). A further desireable feature would be to obtain control over the non-volatile resistance value so as to obtain any desired intermediate value.
In addition to the binary, non-volatile resistive stable states, if the device provides a continuous (analog) control over its non-volatile (resistance) value, it would form an ideal component (element) for a variety of non-volatile control operation in adaptive (feedback) circuitry. Furthermore, as a synapse in electronic (artificial) neural networks, it would provide an additional "dimension" for analog information storage and processing.